The invention concerns a machine for sheet-fed rotary printing or sheet coating comprising the features of the independent claim.
Corona treatment involves electric flaming at a high voltage. The ionised air between the electrodes corrodes the surface of the plastic material in response to ozone and oxygen, i.e. oxidation. Polar groups are generated on the plastic surface which are easy to wet.
In practice, corona treatment has advanced to one of the most frequently used methods for surface refinement. It increases the surface tension of polymeric films and foils, aluminium foils, laminated paper and cardboard as well as sandwiched materials before they are printed, glued or coated. This surface treatment improves adhesion between the layers to be connected or adhesion of the coating substance or color. The final effects of the treatment depend on various parameters such as the power of the electrodes, duration of discharge, type of material treated, temperature and moisture, possible impurities, surface morphology and mainly the minimum gap width.
The mechanical construction must be robust. The electrode segments must maintain their settings despite vibration of the system and other unfavorable conditions.
JP 591 31 460 A discloses a device for treatment of sheet-like substrates. However, full-surface corona treatment does not require precise, registered transport and therefore no gripper technology is used for the sheet transport. The flat sheets are transported on belts using vacuum and without grippers. Setting of the gap width is therefore straightforward, resulting in stable handling.
DE 100 41934 discloses a corona device which is used after the last printing mechanism. This reference does not address the problem of minimum gap width, since the gap width is less critical for this varnish coating of printed paper. This corona device is used at a completely different location than in the present invention.
Corona treatment of sheet-like materials in the printing industry is an established technology (DE 199 10 740, DE 198 43 955, DE 101 06 385). The gap width of a few millimeters includes the thickness of the substrate sheet plus a gas gap above the substrate surface to be treated.
The sheet-like substrates require gripper elements for exactly registered transport whose gripper backs must therefore be disposed several millimeters above the substrate surface. The gap width above the substrate surface to be treated is therefore too large for optimum corona treatment.
DE 100 39 073 represents the principal prior art and main reference point, wherein, despite the use of grippers, an optimum gap width is nevertheless obtained by providing a lifting device or gripper recesses.
The substantial disadvantage of this arrangement is that a lifting motion at high speeds is a problem per se and also includes the risk of vibrations. A drum with gripper recess is format-related and therefore relatively large and expensive due to additional insulation.
Moreover, drums of such large size cannot be integrated in the first printing or coating mechanism due to access problems.
A sheet acceleration path is required between the stacked feeder and the first printing mechanism. The printing cylinder moves constantly at the printing speed. The supply system must precisely guide the sheet, bring it up to the printing speed, and transfer it to the grippers of the printing cylinder.
This object has been achieved in many machines using pivoting grippers, disposed above or below the sheet.
Alternatively, a so-called ranger drum, stop drum system or suction drum system can be used. None of the above-mentioned feed systems can accommodate surface refinement systems, so-called e.g. corona treatment systems, for reasons of space.
These sheet substrates can be pre-treated in a separate working step, which involves additional cost and time. Moreover, the corona charge loses its effectivity with time.
In accordance with a press report by the company König and Bauer in “Deutscher Drucker” dated 13 Nov. 2003, the printability of e.g. plastic materials can be improved by installing an additional upstream corona tower. This arrangement requires great expense and space (grosso modo 75% of the price of a printing mechanism).
Alternatively, the printing material can be coated with a primer as primary coating, which necessitates an additional printing mechanism and associated dryers in the printing machine (see DE 100 04 997 A1). Printing machines having such equipment are therefore expensive.
It is the underlying purpose of the invention to develop a novel sheet supply system, with appropriate geometry, to exactly guide and bring the sheet up to the subsequent printing speed and to transfer it to the grippers of the printing cylinder, wherein the front side and/or the back side of the sheet is/are subjected to surface treatment, e.g. corona treatment, and without additional upstream drums, transfer cylinders or printing mechanisms.